One of my kittens was diagnosed of FIP which is unfortunately incurable. He basically had been losing a lot of weight, stopped eating, became super lethargic, and finally starting throwing up every few hours. It was so hard watching him go through so much pain.
He had no options left but to bear the pain until his end so my husband and I made the very difficult decision of putting him to sleep; it would be a lot easier for him than living in misery for his few days/weeks left of life.
I only had the little kitten for a month and it was so hard dealing with the loss. Driving away from the vet I had tears streaming down my face when I finally realized that this is just how life is. We're all only on this earth for a short amount of time and then one day the time for each and every one of us will come. And who knows, maybe it'll be tomorrow...or even today. Lucky for animals, they're not accountable for their lives. We, on the other hand, don't have it that easy.
God says in the Qur'aan:
كُلُّ نَفْسٍ ذَائِقَةُ الْمَوْتِ ۗ وَإِنَّمَا تُوَفَّوْنَ أُجُورَكُمْ يَوْمَ الْقِيَامَةِ ۖ فَمَن زُحْزِحَ عَنِ النَّارِ وَأُدْخِلَ الْجَنَّةَ فَقَدْ فَازَ ۗ وَمَا الْحَيَاةُ الدُّنْيَا إِلَّا مَتَاعُ الْغُرُورِ
Every soul will taste death, and you will only be given your [full] compensation on the Day of Resurrection. So he who is drawn away from the Fire and admitted to Paradise has attained [his desire]. And what is the life of this world except the enjoyment of delusion. [3:185]
Life is short and it's purpose is not to waste it doing aimless things and being deluded by the glitter of this world. Its so easy for us to be so distracted by the tangible worldly things in life from our true purpose: to please God, do good, and hope to enter Paradise.
My kitten's death was the first time I ever had to experience death first hand and face it in front of me. As emotionally difficult as it was, there was a great lesson in it for me.
We need to make sure we're not wasting our time and really refocus our purpose and our goals in life because God knows, life is short.